Oil wells and many other types of wells often comprise a well bore lined with a steel casing. A casing is a string of pipes that are threaded at each end to be interconnected by a series of internally threaded pipe couplings. A lower end of the casing is perforated to allow oil, water, gas, or other targeted fluid to enter the interior of the casing.
Disposed within the casing is another string of pipes interconnected by a series of threaded pipe couplings. This internal string of pipes, known as tubing, has a much smaller diameter than casing. Fluid in the ground passes through the perforations of the casing to enter an annulus between the inner wall of the casing and the outer wall of the tubing. From there, the fluid forces itself through openings in the tubing and then up through the tubing to ground level, provided the fluid is under sufficient pressure.
If the natural fluid pressure is insufficient, a reciprocating piston pump is installed at the bottom of the tubing to force the fluid up the tubing. A reciprocating drive at ground level is coupled to operate the pump's piston by way of a long string of sucker rods that is driven up and down within the interior of the tubing. A string of sucker rods is typically comprised of individual solid rods that are threaded at each end so they can be interconnected by threaded couplings.
Since casings, tubing, and sucker rods often extend thousands of feet, so as to extend the full depth of the well, it is imperative that their respective coupling connections be properly tightened to avoid costly repair and downtime. Couplings for tubulars (i.e., couplings for tubing and casings), and couplings for sucker rods (referred to collectively herein as “rods” or “sucker rods” are usually tightened using a tool known as tongs. Tongs vary in design to suit particular purposes, i.e., tightening tubulars or rods, however, each variety of tongs shares a common purpose of torquing one threaded element relative to another. Tongs typically include a hydraulic motor that delivers a torque to a set of jaws that grip the element or elements being tightened.
Various control methods have been developed in an attempt to ensure that sucker rods are properly tightened. However, properly tightened joints can be difficult to consistently achieve due to numerous rather uncontrollable factors and widely varying specifications of sucker rods. For instance, tubing, casings and sucker rods each serve a different purpose, and so they are each designed with different features having different tightening requirements.
But even within the same family of parts, numerous variations need to be taken into account. With sucker rods, for example, some have tapered threads, and some have straight threads. Some are made of fiberglass, and some are made of steel. Some are one-half inch in diameter, and some are over one inch in diameter. With tubing, some have shoulders, and some do not. Even supposedly identical tongs of the same make and model may have different operating characteristics, due to the tongs having varying degrees of wear on their bearings, gears, or seals. Also, the threads of some sucker rods may be more lubricated than others. Some threads may be new, and others may be worn. These are just a few of the many factors that need to be considered when tightening sucker rods and tubulars.
Furthermore, as tongs system components age, their ability to react consistently is reduced. For example, the amount of energy, in the form of hydraulic pressure, necessary to generate a specific torque on an elongated member by a tongs drive increases over time. Also, the amount of speed generated on an elongated member by a tongs drive based on a constant current level transmitted to the hydraulic valves in the tongs drive system decreases over time as components wear out. Because the system does not react consistently over time, it is difficult to develop a static system that can effectively tighten elongated members over the life of the tongs.
In addition, one main feature of a tongs control system is to be able to make up a rod connection to a specific pre-programmed circumferential displacement based on rod parameters, such as manufacturer, grade, and size. To have the joint connection stop at exactly the correct circumferential displacement value, the controller must issue a “stop” command to the system at a slightly earlier time than desired, to account for the slight delay in system response (electronic component delay, hydraulic component delays, mechanical drive train, rotational inertia). The problem is that this time delay between the stop command being issued and the rod actually stopping is quite short, on the order of 10 milliseconds, and is influenced by changes in temperature. One variation is due to changes in viscosity of the hydraulic fluid. As temperature of the hydraulic fluid increases, viscosity decreases, and the tongs motor is less efficient (conveys less torque, or energy for given flow and pressure). Higher temperatures result in shorter stopping times than when the hydraulic fluid is cold, viscosity is high, and more “sluggish” behavior is seen. Mechanical friction also varies with temperature. This shows up in the response time of the two spools in the hydraulic valve, the tongs motor, and drive mechanism. In this case, hotter temperatures tend to “open up” the devices, and this reduced friction provides faster response times.
Consequently, a need exists in the art for a system and method for evaluating system efficiency in order to know when components are not operating up to acceptable levels. In addition, a need exists in the art for a system and method for monitoring temperature fluctuations both internal and external to the system and modifying the time delay for generating stop signals in order to ensure proper tightening of the rod or other elongated member. Furthermore, a need exists in the art for a system and method for comparing current connection failure levels to historical connection failure levels to determine if improvement has been achieved.